A farmer driving a tractor in his fields near the Gaza Strip said Palestinian gunmen fired at him on Sunday afternoon, putting security forces in the region on alert as the cease-fire appeared to have been breached yet again. The man based his claim on the fact that one of the tractor's windows had suddenly shattered. He himself was not wounded. Some time later, however, army officials downplayed the report, saying that there was no evidence of bullets hitting the tractor. They added that the glass may have been hit by a rock which had somehow been thrown at the window after being caught up in the machinery. The incident occurred shortly after Israel reopened the Erez, Sufa and Nahal Oz crossings into Gaza on Sunday morning, following closure of the crossings in response to a Kassam attack on Thursday. Meanwhile, Israel warned Saturday that any delay by Hamas on contacts over a prisoner swap involving the release of captured IDF soldier Gilad Schalit may torpedo the Egyptian-mediated Gaza Strip truce agreement reached last month. The warning followed an announcement by Hamas on Friday that it was considering suspending talks over the release of Schalit because Israel was not abiding by the cease-fire. But an Israeli official told The Jerusalem Post that Hamas was already violating the terms of the truce by permitting sporadic hostile fire from Gaza into Israel, and that violating the Schalit clause could torpedo the whole deal. Mark Weiss and Khaled Abu Toameh contributed to this report.